I recently read something that was rather interesting...
In 1944, Averell Harriman, then our Ambassador to the USSR, said in September 1944 to Franklin Roosevelt tha "Unless we take issue with the present policy, there is every indication that the Soviet Union will become a world bully wherever their interests are involved." At the time, FDR disagreed, and felt he could 'control' Stalin and, by making concessions, placate Stalin. Churchill agreed at the time, stating that while Chamberlain was wrong in trusting Hitler, he (Churchill) felt he wasn't wrong in trusting Stalin.
However, in April 1945, in one of the last memos before his death, after Yalta, FDR had come to agree with Harriman's assessment, stating: "Averell is right. We can't do business with Stalin. He has broken every one of the promises he made at Yalta."